The present invention relates to the metallization of non-conductive surfaces, more particularly to the metallization of non-conductive surfaces in the course of manufacture of printed circuits, and still more particularly to the electroless metallization of non-conductive through-hole surfaces in double-sided or multi-layer printed circuit boards.
The art long has been familiar with the desirability of providing a metallized coating on non-conductive surfaces for functional and/or aesthetic purposes. A particularly important technological area where the techniques of metallization of non-conductive surfaces have found applicability is in the manufacture of printed circuit boards wherein metallization is used to provide patterned, conductive circuitry on non-conductive (insulating, dielectric) substrate materials.
Within the area of printed circuit board manufacture itself, metallization of non-conductive surfaces may come into play at a number of steps in the overall process. One particular area of substantial import is the electroless metallization of the non-conductive surfaces of through-holes.
In the manufacture of printed circuits, it is now common-place to provide planar boards having printed circuitry on both sides thereof. Of increased importance are so-called multi-layer circuit boards, comprised of laminates of non-conductive substrate and conductive metal (e.g., copper), wherein one or more parallel innerlayers or planes of the conductive metal, separated by non-conductive substrate, are present within the structure. The exposed outer sides of the laminate contain printed circuit patterns as in double-sided boards, and the inner conductive planes may themselves comprise circuit patterns.
In double-sided and multi-layer printed circuit boards, it is necessary to provide conductive interconnection between or among the various layers or sides of the board containing conductive circuitry. This is achieved by providing metallized, conductive through-holes in the board communicating with the sides and layers requiring electrical interconnection. The predominantly-employed method for providing conductive through-holes is by electroless deposition of metal on the non-conductive surfaces of through-holes drilled or punched through the board.
As is well known in the art, electroless deposition of metal onto non-conductive surfaces requires that a material catalytic to the electroless depositing reaction be present on the non-conductive surfaces. In the typical processes relevant to printed circuit board manufacture, wherein through-hole metallization with copper is employed, the catalytic material comprises palladium metal. The process of applying catalytic material to the substrate surfaces, known generally as "activation", most typically involves contact of the substrate with a true or colloidal solution of palladium and tin compounds (generally, chlorides) as described, e.g., in U.S. Pat. Nos. 3,011,920 and 3,532,518. It is generally believed that the tin compounds act as protective colloids for the catalytic palladium. In many cases the activation is followed by an "acceleration" step which serves in some manner to expose (or increase exposure of) the active catalytic species, although activating baths are known which do not require a separate acceleration step. Following provision of catalyst in this manner on the non-conductive surfaces, the surfaces are then contacted with an electroless metal (copper) depositing bath in which catalyzed chemical reduction reactions lead to deposit of metal from the bath onto the catalyzed surfaces.
In utilizing electroless depositing technology for provision of conductive coatings on through-hole surfaces, the patent literature often speaks only generally of the adherence of the catalytic material to the non-conductive surfaces of the through-holes, many times teaching or inferring that the matter is one only of providing sufficient roughening of the surfaces (as might be achieved simply in the hole-drilling process) to promote catalyst adherence. In the practical art of circuit board manufacture, however, the need for complete through-hole coverage with conductive metal (and hence the need for complete catalyzation of the non-conductive through-hole surfaces) is so acute that additional measures usually are taken. One such approach is the process known as "conditioning".
It generally is found that notwithstanding the fact that the topography of the through-hole surfaces can be such (e.g., roughened, pitted) as to promote adhesion of catalyst for electroless metal deposition, the properties of the non-conductive substrate material per se may lead to poor adhesion. A primary example of this is found in the glass-filled epoxy resins which are used extensively in the industry as non-conductive substrate material in printed circuit boards. Glass fibers have been shown to adsorb palladium activating material only poorly, leading in turn to poor (incomplete or too thin) coverage of subsequently electrolessly deposited copper in the through-holes. A possible explanation for this experience is that the glass fibers tend to have a highly negative surface charge and do not attract the typical tin-palladium catalyst particles which also carry negative charge (e.g., due to chloride ion). This experience, as may be expected, also is found with other glass-filled substrates. However, the problem of poor metal coverage in through-holes is not restricted to glass-containing non-conductive substrates, and exists even in those cases where the substrate is composed of any number of a variety of typical, non-glass-containing, non-conductive materials used as circuit board substrates.
In particular response to the problems of poor metal coverage found with epoxy-glass substrates, the art developed printed circuit manufacturing processes which employed a conditioning step preparatory to activation of through-hole surfaces. The conditioning agents chosen were those which function to improve the adsorption of activating material on the glass fiber surfaces and to improve subsequent electroless plate quality, and typically were of the cationic film forming class of compounds. The exposed through-hole surfaces (e.g., epoxy, glass fibers and, for multi-layer boards, edges of copper innerlayers) were thus coated with a film and the catalytic species (and ultimate electroless metal coating) then essentially adhered to and built up on the film.
Although conditioned through-holes result in superior metal coverage therein as compared to non-conditioned through-holes, coverage of through-holes in glass-epoxy substrates using early conditioning technology was far from perfect, particularly as to coverage on the ends of the glass fibers. Improved technology in conditioners has focused on choice of conditioning agents and/or operating parameters which provide for better metal coverage.
While complete coverage of through-holes with metal is essential, it must also be recognized that coverage in and of itself cannot be an end all and be all in any metallization process, and particularly in printed circuit board manufacture. The ultimate criterion for success is that the metallized through-hole retain its integrity throughout the entire board manufacturing process, throughout all operations subsequently conducted on the board (e.g., attachment of components, etc.) and throughout all phases of use of the board. As the art moves progressively towards focus on conditioning through-holes to provide films thereon which completely coat exposed through-hole surfaces, a serious problem is introduced in terms of the integrity of the electroless metal coating. Although not wishing to be bound by theory as such, it is believed that a situation arises where the distance of the metal coating from the actual through-hole surface becomes greater and greater due to the increasing thickness of the conditioning film therebetween. When this distance reaches a certain point, the metal deposit is actually more closely associated with (carried by) the film than with the through-hole surfaces coated by the film. In the processes involved in manufacture and use of the board, the film itself becomes a potential area for failure and can lead to loss of adhesion of the metal in the through-hole, blistering and the like. In multi-layer circuit boards, poor metal adhesion in the through-holes also evidences itself via poor adhesion of the metal to the exposed metal innerlayer surfaces in the through-hole.
Another problem associated with the current art of through-hole conditioners, particularly, it is believed, in the choice of agents based primarily upon their thick film-forming characteristics, is the fact that these films also will, of course, coat all areas of the board, not just through-holes, since the conditioning process involves immersion of the entire board in the conditioning solution. For example, copper-clad substrates with through-holes will, after conditioning, also have the conditioning film on the copper foil surfaces. While the presence of conditioning agents on such surfaces is not per se undesirable and may, indeed, be beneficial in promoting deposited metal adhesion on these surfaces, the known conditioning agents generally leave too thick a coating or film on these surfaces which may result in the films becoming an undesirable barrier to metallization adherence and/or conductivity. For this reason, it is necessary in the art to subject the board to a micro-etching step after conditioning to remove at least some of the conditioning agent from the copper foil surfaces. Although micro-etching will in any event be practiced with copper-clad boards in a printed circuit manufacturing process to remove oxides from the copper surfaces, the foregoing problem imposes on the circuit board manufacturer the constraint of being required to perform the micro-etch after the conditioning step rather than earlier in the process, which, for some processes, would be more economical. Moreover, in the required sequence, conditioning agents gradually will contaminate the micro-etch solution thereby limiting its useful operating life.